1. Field of the Invention
The present invention relates to a finder optical system employed in a camera. More particularly, the present invention relates to a finder optical system useful for a camera which constitutes a trimming system.
2. Prior Arts
In the trimming system, an information specifying an area A including the center of an image formation area B on a film 50 shown in FIG. 1a is memorized onto a predetermined portion 51 on the emulsion surface of the film 50. The area can be enlarged to be printed in the area A' on photographic paper 52 shown in FIG. 1b by using the information memorized onto the portion 51. To be more specific, at a trimming mode photographing, the trimming information is taken onto the predetermined portion 51 of a film in the form of a code signal by a code taking unit; the information is detected and read by a reader unit in printing. The area A is magnified by a zooming operation of an optical system of a printer according to the information. As shown in FIG. 1c, in order to take the trimming information onto the film 50, light emitting diodes LED1, LED2, and LED3 are provided which are controlled by the code signal. The light emitted by the diodes LED1 to LED3 is led to the predetermined portion 51 of the film 50 through optical fibers 53, 54, and 55 and the predetermined portion 51 is exposed by the light.
For a camera to which such trimming system is applicable, a finder by which the trimming area (the area A) is seen in an adequate large size at a trimming photographing mode is more convenient than a finder by which the trimming area is seen in small size. It is therefore desirable to fulfill a field frame of the finder with an image corresponding to the trimming area without changing the size of the field frame actually seen in the finder window.
It is hereinafter referred to as "pseudo zooming" to enlarge a trimming area to the predetermined size of the field frame of a finder. On the other hand, when a taking lens system has a zooming function, a finder optical system has to perform a zooming operation in response to the zooming operation of the taking lens system. This zooming operation of a finder optical system is hereinafter referred to as "optical zooming". Many of conventional finder optical systems perform only the optical zooming operation with an objective lens system.
In a real-image-type finder optical system, an image formed by an objective lens system is inverted upside down and rightside left, and therefore if it is viewed only through the object lens system and an ocular, it will seen as the inverted image. Therefore, image erecting system are usually provided between the objective lens system and the ocular. As the image erecting system, a Porro prism has been used in conventional finder optical systems.
When the above-described optical zooming operation and pseudo zooming operation are performed in a finder optical system, the total magnification of the two kinds of zooming operation increases (e.g. when the magnification of an optical zooming is 3.times. and that of a pseudo zooming is 2.times., the total magnification is 6.times.). Therefore, when the optical and pseudo zooming operation is performed only by an objective lens system, the total length of the finder optical system becomes longer, which makes it impossible to realize a compact camera body. That is, since a finder magnification V is expressed by the following equation: EQU V=f1/f2,
where f1 is a focal length of an objective lens system and f2 is a focal length of an ocular, in order to increase the magnification V, the focal length f1 of an objective lens system is increased. However, this also increases an axial length (length along the optical axis) of the objective lens system, which naturally increases the total length of the finder optical system.
The magnification V is increased also by decreasing the focal length f2 of the ocular. To decrease f2, the ocular must be arranged close to the image plane where the image is formed by the objective lens system. However, in a conventional finder optical system where the Porro prism is employed, the focal length f2 of the ocular cannot be decreased so much since the ocular touches the Porro prism.
A finder optical system in which a relay lens system is used for erecting the inverted image is also known. However, with respect to a finder optical system employing a relay lens system, it has not been fully examined which optical system should perform a pseudo zooming operation and which optical system should perform an optical zooming operation.